CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION--continued.

5. SUMMARY OF THE LAST AND PRESENT CHAPTERS.

In these chapters I have endeavoured to show that if we make due allowance
for our ignorance of the full effects of changes of climate and of the
level of the land, which have certainly occurred within the recent period,
and of other changes which have probably occurred--if we remember how
ignorant we are with respect to the many curious means of occasional
transport--if we bear in mind, and this is a very important consideration,
how often a species may have ranged continuously over a wide area, and then
have become extinct in the intermediate tracts--the difficulty is not
insuperable in believing that all the individuals of the same species,
wherever found, are descended from common parents. And we are led to this
conclusion, which has been arrived at by many naturalists under the
designation of single centres of creation, by various general
considerations, more especially from the importance of barriers of all
kinds, and from the analogical distribution of subgenera, genera, and
families.

With respect to distinct species belonging to the same genus, which on our
theory have spread from one parent-source; if we make the same allowances
as before for our ignorance, and remember that some forms of life have
changed very slowly, enormous periods of time having been thus granted for
their migration, the difficulties are far from insuperable; though in this
case, as in that of the individuals of the same species, they are often
great.

As exemplifying the effects of climatical changes on distribution, I have
attempted to show how important a part the last Glacial period has played,
which affected even the equatorial regions, and which, during the
alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. As showing how
diversified are the means of occasional transport, I have discussed at some
little length the means of dispersal of fresh-water productions.